This invention relates to a subsystem in an automatic document feeder for feeding of sheet media. More particularly, this invention relates to a subsystem in an automatic document feeder suitable for use with a scanner wherein a scanned sheet is moved back and forth during scanning.
The throughput of office equipment such as photocopiers, scanners and fax machines is measured by the number of sheet media that can be processed by these equipment. One factor that determines the throughput of these equipment is the throughput of an automatic document feeder that is usually attached to these equipment for feeding sheet media to these equipment for processing. The throughput of the automatic document feeder should be as high as that of the equipment so as to make the most efficient use of the equipment.
Currently, the throughput of some scanners is limited by the throughput of an automatic document feeder that is attached to the scanners. In these scanners, the complicated back and forth transportation of sheet medium allows sheet media to be processed only one at a time. The automatic document feeder is designed to be able to pick and feed a new sheet medium only after the preceding sheet medium is fed through to the end of a document path. Such a method of sheet processing is inefficient and greatly limits the throughput of the document feeder. End-to-end feeding of the sheets is used in higher-end scanners to increase throughput. In end-to-end feeding, as soon as a sheet medium leaves a media tray, the next topmost sheet medium is picked and advanced to closely follow the preceding sheet medium.
Such end-to-end feeding of sheet media is easily accomplished in the high-end scanners. These high-end scanners usually have an abundance of memory for capturing scanned image of an entire sheet medium in one continuous pass of the sheet medium. However, low-cost units do not have the luxury of such a large memory. To completely capture the image on a sheet medium, each sheet medium is divided into contiguous sections. The size of each section is determined by the amount of available memory in the units. Each section of the sheet medium is separately scanned and the data of the scanned image on the section is stored in the available memory for uploading to a computer. Once the data in the memory is uploaded, the memory becomes available to capture another set of data.
The quality of a scanned sheet depends on several factors, one of which is the speed at which the sheet medium is passed over a scan zone of a scanner. To achieve a higher quality scanned image, the sheet medium is advanced over the scan zone at a uniform speed. In order to get a subsequent section across the scan zone at a uniform speed after a section is scanned, the sheet medium will have to be retracted, or reversed, in the document path. This reversing of sheet medium is required to allow the sheet medium to be accelerated to reach the uniform speed when the subsequent section reaches the scan zone. Such forwarding and reversing of a sheet medium poses a challenge in the picking and advancing of the sheet medium in a low-cost document feeder as illustrated in FIGS. 1A and 1B.
FIGS. 1A and 1B show a media feed subsystem 2 of an automatic document feeder wherein a single motor (not shown) is used to drive both a pick roller 4 and a drive roller 6. The motor drives a main gear 8, which in turn drives the pick and drive rollers 4, 6 via free gears, generally illustrated in the figures as free gears 10, 12. FIG. 1A shows that the main gear 8 being driven in a counterclockwise direction to advance a first sheet medium 14 in a forward direction as indicated by arrow F. The first sheet medium 14 is shown 30 to have been advanced beyond the pick roller 4. Using an appropriate gear ratio the drive roller 6 is driven faster than the pick roller 4 to create a gap 16 between a trailing edge 18 of the first sheet medium 14 and the leading edge of a second sheet medium 20. FIG. 1A shows the second sheet medium 20 being urged against the pick roller 4 for picking. Driving the main gear to reverse the drive roller 6 would also cause pick roller 4 to reverse. This reversing of the pick roller 4 would push the second sheet medium 20 away from under the pick roller 4 as shown in FIG. 1B. There is thus a tendency during subsequent forwarding of the pick roller 4 that a third sheet medium is pick in place of the second sheet medium, thereby disrupting the sequence of media feeding.
This problem is solved by the introduction of a slip clutch (not shown) that is attached to a shaft (not shown) of the pick roller 4 to allow unidirectional rotation of the pick roller 4. FIG. 2A shows a subsystem similar to that in FIGS. 1A and 1B with the addition of such a slip clutch. When the drive roller 6 is reversed, the slip clutch prevents the pick roller 4 from rotating in the reverse direction so that the pick roller 4 continues to bear upon the second sheet medium 20. This solution creates another problem. As the drive roller 6 and pick roller 4 are driven in the forward direction, there is a tendency for the two sheet media 14, 20 to overlap if the first sheet medium 14 is reversed by a amount that the differential in speeds of the two rollers 4, 6 cannot correct. The overlapping of sheet media is unacceptable because image on the overlapped portion cannot be scanned and the scanner is also unable to easily identify the sheet media boundaries. The pre-selection of gear ratio to ensure a speed differential to cater to the worst case scenario would compromise throughput as the gap between two sheet media could be large.
The prior art therefore creates the need for a method and apparatus for sequencing sheet media to increase throughput in a scanner or the like without increasing the cost of prior art apparatus excessively.
In one aspect of the present invention, a method of driving a sheet media feed subsystem for substantially end-to-end advancement of sheet media in an automatic document feeder suitable for use with a scanner involves driving a drive roller to advance a sheet medium section-by-section across a scan zone of the scanner. In advancing a section of the sheet medium across the scan zone, the method involves accelerating the drive roller in a forward direction to attain a substantially uniform speed when the section reaches the scan zone. Thereafter, the drive roller is driven at the substantially uniform speed to advance the section across the scan zone. When the section is scanned, the drive roller is decelerated to bring the drive roller to a stop. The drive roller is then reversed to reverse the sheet medium by a predetermined distance so as to allow the drive roller to be subsequently accelerated to bring a next section to the substantially uniform speed when the next section reaches the scan zone. As the drive roller is reversed, a pick roller is prevented from reversing. As the drive roller is driven in the forward direction following a reversed rotation, the pick roller is similarly driven in the forward direction after a predetermined period. When a preceding sheet medium leaves the pick roller, the pick roller picks and advances a next sheet medium to follow the preceding sheet medium closely to leave a gap between the two sheet media. The delay in driving the pick roller prevents overlapping of the two sheet media.
In another aspect of the present invention, a media feed subsystem according to a preferred embodiment that is suitable for implementing the method above has a drive roller, a pick roller, a slip clutch and a drive system. The drive and pick rollers are driven by the drive system according to the method above. The slip clutch allows the pick roller to be driven in a single direction and prevents the pick roller from reversing when the drive roller is reversed. The drive system includes a delay mechanism that allows the pick roller to be driven after the drive roller is driven forward for a predetermined period following the reversing of the drive roller.
Preferably, the drive system includes a single motor that drives the pick roller and the drive roller via a gear train. The delay mechanism preferably includes a delay gear for driving the pick roller. This delay gear is attached to a second shaft to allow the second shaft to rotate in one direction to engageably drive the delay gear and to rotate in the other direction to disengage the delay gear to delay its driving by the predetermined period.
Preferably, the delay gear has an aperture for receiving the second shaft. The delay gear has a gap defined therein for receiving a stub that is fixedly attached to the second shaft. The movement of the stub within the gap allows the second shaft to be rotated without rotating the delay gear. However, when the second shaft is rotated to allow the stub to engage a wall that defines the gap, further rotation of the second shaft will also cause the delay gear to rotate.